The fabrication of silicon integrated circuits typically includes one or more steps of forming layers of silicon dioxide, having a general composition of SiO2, although some variation in its stoichiometry is possible. In some applications, dopants are added. For brevity, this material may hereafter be referred to as oxide. Silicon dioxide is a rugged material that bonds well with silicon and is electrically insulating, that is, dielectric. Thicker layers of oxide are typically deposited by spin-on glasses or by chemical vapor deposition, particularly when they form inter-level dielectric layers, which may be formed over metal and other oxide features. However, thin oxide layers formed over silicon may be formed by oxidizing the silicon to form silicon oxide. The silicon to be oxidized may be monocrystalline silicon of the wafer or polysilicon deposited as a layer on the wafer in a multi-level structure. Gate oxide layers may be formed by oxidation of typically about 1 nm or less. Pads and STI (shallow trench isolation) liners may similarly be formed to thicknesses of typically 5 to 10 nm. The oxide layer not only electrically insulates the underlying silicon but also passivates the silicon/dielectric interface.
Oxidation is conventionally performed by heating the silicon surface to approximately 1000° C. to 1200° C. or higher and exposing it to gaseous oxygen for dry oxidation or to steam (H2O) for wet oxidation. Such thermal oxidation may conventionally be performed in a furnace accommodating large number of wafers, but furnaces have in part been superseded by single-wafer processing chambers utilizing a process called rapid thermal oxidation (RTO), a form of rapid thermal processing (RTP). In RTO, high-intensity incandescent lamps rapidly heat a silicon wafer to very high temperatures and oxygen is flowed into the RTP chamber to react on the surface of the hot wafer to react with the silicon and produce a layer of silicon oxide on top of the wafer. Gronet et al. disclose oxidation in an RTP chamber in U.S. Pat. No. 6,037,273, incorporated herein by reference in its entirety. One advantage of RTO is that the walls of the RTP chamber are typically much cooler than the wafer so that oxidation of the chamber walls is reduced. Gronet et al. disclose injecting oxygen and hydrogen gases into the RTP chamber to react near the hot wafer surface for in situ generation of steam.
It has been recognized that oxygen radicals O* provide several advantages in silicon oxidation. The oxygen radicals more easily react than oxygen gas so that the oxidation rate is increased for a given temperature. Further, the radicals promote corner rounding, an important feature in STI.
Oxygen plasmas have been used for oxidation, but they are felt to subject the semiconducting silicon and dielectric layers to damage particularly when the oxygen species is charged, e.g. O— or O═.
Ozone (O3) is an unstable form of oxygen gas that may be considered an oxygen radical since O3 spontaneously dissociates into O2 and O*, particularly when exposed to surfaces held at temperatures of greater than 400° C. It is known to use ozone in silicon oxidation, see U.S. Pat. No. 5,294,571 to Fujishiro et al. and U.S. Pat. No. 5,693,578 to Nakanishi et al. However, most known prior art for ozone-assisted oxidation occurs at relatively high temperatures and low ozone concentrations.
Another approach for low temperature oxidation supplies the reactor chamber with a gas mixture of oxygen gas O2 and ozone O3, as disclosed in U.S. Pat. No. 5,330,935 to Dobuzinsky et al. (hereafter Dobuzinsky). Ozone is a metastable form of oxygen that may be generated in a microwave or UV generator and which readily dissociates into O2 and the oxygen radical O*. Dobuzinsky supplies the ozone-rich mixture into a thermal reactor operated at a relatively low temperature but including additional RF plasma excitation of the ozone. However, Dobuzinsky's reactor is still a hot-wall reactor so that the ozone quickly dissociates inside the chamber and equally reacts with the chamber walls. Dobuzinsky does however mention the possibility of RTO after their plasma oxidation.
More recent technology has imposed different constraints upon silicon oxidation processes. In view of the very thin layers and shallow doping profiles in advanced integrated circuits, the overall thermal budget and maximum processing temperatures are reduced. That is, the typical oxidation temperatures of greater than 1000° C. are considered excessive even when used with the rapid temperature ramp rates available in RTP. Furthermore, the gate oxide thickness are decreasing to well below 1 nm, for example, 0.3 to 0.6 nm in the near future. However, to prevent dielectric breakdown and increase reliability, the gate oxides must be uniformly thick and of high quality. Plasma oxidation may be a low temperature process because it produces oxygen radicals O* which readily react with silicon at low temperatures. However, charging and other effects on the fragile thin oxide prevent plasma oxidation from being widely adopted. The fabrication of advanced integrated circuits is not only constrained by a reduced thermal budget, they it is also facing decreasing limits in the maximum temperature to which the ICs may be exposed even for short times. The known prior art of ozone oxidation does not satisfy the more recent requirements.
It is felt that the prior art insufficiently utilizes the advantages of ozone for low temperature oxidation without the use of plasmas.
Furthermore, ozone is considered explosive. Safety concerns are greatly alleviated if the pressure within a chamber containing ozone is held at a pressure of no more than 20 Torr. Such low pressures, however, disadvantageously decrease the oxidation rate.